Ever since the advent of root canal therapy, a need has existed to reinforce and replace missing tooth structure. A tooth, after performance of root canal therapy, is more brittle than a live tooth. Most often its morphology is compromised and, given that its blood supply no longer exists, the tooth is more susceptible to fracture. Root canal therapy, is often required on account of pulpal trauma. Also, when a tooth has fractured horizontally to the point where remaining tooth structure can no longer support a restoration, root canal therapy becomes an important option to consider in providing more vertical support, albeit internal, for a restoration.
After root canal treatment the tooth is, in its remaining anatomy and structural support, analogous to a hollow tube in need of 1) internal support, and 2) a method of securing a coronal prosthesis. To put it another way, if you liken a post treatment tooth and the coronal prosthesis to a pair of hollow tubes, then the post would be the dowel that would be inserted through both tubes to hold them together.
From an engineering perspective, the length of post inside the tooth should exceed the length of the restoration to be placed on it. The fulcrum on the post created with lateral forces should exist within the tooth and the moment created with lateral forces should be less at the fulcrum point externally than internally. A post should enhance the mechanical properties of the tooth, and it should react to stresses similarly. It should allow the completely restored construct to function like a natural tooth, and ideally, be the same, or nearly the same, color.
Principal design criteria for clinically effective post and pin holes are prior art in the field of dentistry. Choosing the best canal, utilizing optimal tooth structure to maximize surface area, is also known to dentistry, and those schooled in the art of the restoration of candidate teeth.
Present day posts and pins fall into two categories: cast and prefabricated. The invention is primarily concerned with prefabricated posts, although it can be applied to cast posts. Indeed, cast posts have their own design criteria and sometimes they are the only posts that can be created when limiting factors exist. Such limiting factors include, but are not limited to, cross-sectional shape and diameter of root, and root curvature. Such cast posts may be treated using the subject concepts of the invention and used in the same way that the invention is applied to prefabricated posts.
Prefabricated posts come in all shapes and sizes, however, for the most part, they are made from metal materials such as stainless steel or other cobalt chrome alloys titanium, gold or brass.
As is well known in the art, the conventional support post is cemented in the hole in the tooth left after the performance of the root canal therapy. In most cases, the hole is substantially cylindrical in shape, this being the shape usually prepared in the root of the tooth after root canal therapy by a dentist's drill. In other cases, the hole is conical in shape, with the apex of the cone pointing toward the apes of the root of the tooth. This shaped hole can also be made by a drill or file. However, these conical holes, with various degrees of taper, are made by drills or files of conical shape which are selected to accommodate a tooth with limited root structure.
After root canal therapy, a hole is established in the root of the tooth. The hole is oversized with respect to the post by approximately 0.025 mm and a post with a base portion matching the shape and size of that hole is selected and cemented into the hole. After the post is cemented in the hole, a quantity of filled resin or amalgam is put over the top of the post in order to build up a core for the support of the coronal prosthesis. Such filled resin is selected for its strength properties in thick layers. Typically, such resin is of the dual cure variety, that is a resin which is prepared by mixing two plastic components together and curing them. In the case of a dual cure resin, curing or hardening will occur automatically after a period of time, or curing can be accelerated using a light source of appropriate wavelength (an auto-polymerizing or self-curing resin may also be used). Typically, such resins are filled with fibers or glass.
After the core resin has been deposited and hardened, it is then shaped to form the final core used to support the coronal prosthesis or crown. After shaping, an impression is taken of the finished core and the teeth proximate to it, and the same may be sent to a lab to allow the fabrication at the lab of a suitable coronal prosthesis.
When the coronal prosthesis has been made, it is returned to the dentist who checks to see that it properly matches the adjacent teeth and the core, and then cements it in place permanently. Typically, a two part cement, e.g. zinc oxyphosphate, polycarboxylic acid, glass ionomer, or bis -GMA- luting agent or other cement is used. This cement is of the time cured variety and hardens after a short period of time, securing the crown to the post. As a result of this multi-step process, the finished assembly of the natural root, post, core and crown forms a restored tooth and takes the place of the completely natural tooth. This restoration can be expected to function for many years.
When failure does occur, it typically takes the form of a failure in adhesion at the interfaces between the post and adjacent members of the restored tooth structure. One possibility is that the layer of cement between the base portion of the post and the root will usually fail due to microleakage of cement, causing the post, crown and core to become loose and fall out. Another possibility is that the filled resin dispensed around the top of the post will detach from the top of the post, causing the crown and core to become loose and fall out.
In an attempt to address these problems, numerous artifices have been developed to increase the adhesion between the base of the post and the root of the tooth. Thus, some posts are provided with threads which effectively increase their surface area at the post/root interface. Threads, grooves or other surface macrotexture also provide space for cement to be contained, which may be of particular importance in the case of a cylindrical post, where the space between the outside diameter of the post and the hole in the root may be minimal.
In similar fashion, cylindrical posts may also be provided with axial fluting on their outside surface. Some posts are also provided with threads or fluting. The objective of such surface structures is to increase the surface area of the post available for adhesion and reduce hydrostatic pressure within the cement and thus improve adhesion between the post and the root by providing more surface area for cement between the post and the root. Increasing the surface area of a post also increases the area over which stresses are distributed during chewing and other normal activities. Because stresses are distributed over a larger area, the amount of stress per unit area during any activity is less and, thus, improves the longevity of the cement layer in the dental restoration. Vertical fluting may also be used to reduce hydrostatic pressure in the canal by allowing cement flow out of the top of the post and the bottom of the post.
In similar fashion, the top of the post to which the restoration shall be ultimately attached is often provided with increased surface area through various means in order to improve adhesion between the core and the post.
Notwithstanding the above measures, many restorations fail over time on account of a failure in the cement layer adjacent to the bottom of the post or a failure in adhesion at the top of the post. This occurs because the adhesion between the post and the cement at its bottom or the core material at the top of the post is primarily a mechanical adaptation of the cement to the post. Specifically, the hardened cement used at the bottom of the post forms a separate mechanical member which has adapted to crevices and forms around protuberances in the inside surface of the hole in the root of the tooth. Thus, adhesion between the root and the bottom of the hole can be viewed as the grip provided by interlocking mechanical members, namely, the root and the post bottom, and the layer of cement in between them.
With time, the hardened cement layer between the post and the root, starts to disintegrate, due to repeated mechanical shocks, resulting in loss of structural features which are critical to the maintenance of the interlocking structures that form the interfaces between the cement and the root canal, and between the cement and the post bottom. A breakdown in the integrity of the interlocking structures may result in microleakage at the cement/tooth interface. Microleakage may also be the cause of cement breakdown, which is then exacerbated by mechanical shocks.
While it is possible to improve the strength of the hardened cement by incorporating fiber, titanium alloy particles or other materials into it, such improvement in strength does little to change the retentions within the tooth. The conventional approach to the problem of cementing a post in a root canal has been to closely match the post to the canal in size and shape, and to have as thin a layer of cement as possible. Thus, the cement or luting agent used in this application is selected for its strength in thin layers.
In the fabrication of the core for the support of the coronal prosthesis, a filled epoxy having strength when deposited in thick layers is used. As previously noted, while a filled epoxy material is used for this particular part of a dental restoration, adhesion problems may still remain between the core and the top of the post.